Heater Core Questions
 

I have no heat in my 98 Ram 1500 5.9l. I changed the thermostat and now the truck comes up to temp like it should but just blows luke warm air. Should I bother flushing the heater core or just change it? I've heard these trucks have heater core problems so I don't want to waste my time if flushing it won't help. Winter is almost here!

Sounds like you could have an air pocket.

It had no heat before because the thermostat was stuck open and thr truck never warmed up enough. Now I've been driving it and it comes up to temp. It has not been going down since I bleed the system. I work at a shop and we have a air evacuator.

If it isn't an air pocket I'm thinking a blockage. If the heater core were bad, you'd smell coolant and the windows would be fogging.

I'm just thinking this because of my old Jeep Cherokee. I fought no heat issues for a long time with it. I flushed and back flushed the heater core several times and it got a bit better but still had no decent heat. I ended up putting a new heater core in it and it really good heat after.

I was assuming you guys all replaced them because of low heat but I guess you all had leaks?

OK, so I backflushed the heater core last night and it blows a little warmer heat but it's still too cold. I think I will have to replace the heater core this weekend.

Is there anything else to check? The truck comes up to temp, the blower motor blows lots of air, the heat is warmer the higher the engine is reving, I'm pretty sure the heater door is working fine, I can here it stop on the hot and clod side, kind of a small thud.

Sounds like the next logical step, core replace.

And yes, most get replaced due to leaking.

hold on. replacing the core is big job, and might not be necessary.
check the temperature of your heater hoses to and from the heater core.
if they're good and hot (too hot to hold onto), then you've got good water flow through the core, and its unlikely that a new core will be any different. you may have a blend door problem, where you're not getting the right air flow across the core.

also, if you're t-stat is not a 195*, consider doing that for the winter time.

Both inlet and outlet hoses are hot as hell. I have a new 195* stat in it.

How can I check the blend door?

I had a 1990 Jeep Cherokee that had good flow through the heater core and cool heat, I replaced the heater core in it and got tons of heat. Just throwing that out there too. I'd rather not do the heater core but if I do it will be tommorow.

I think that the most compelling evidence is that it improved with back flushing the core...